1. Field of the Invention
The invention relates to a driving mechanism for use with the pressure contacts of an electric switching mechanism, such as a vacuum circuit breaker wherein the mechanism includes: a drive means for bringing the contacts to an open and closed position; closing and opening spring means for actuating the drive means to initiate such closing and opening, the opening spring means being cocked during actuation by the closing spring means; and a contact pressure spring for applying pressure to the contacts in the closed position.
2. Description of the Prior Art
In drive mechanisms of the aforesaid type, the motion cycle of the mechanism is required to have a duration which is definite and, in some cases, forces which have a definite dependence on the distance travelled. On the other hand, the mechanism is also required to comply with certain other constraints such as, for example, specified outline dimensions, low-impact motion and low power requirements.
Electric switching mechanisms which employ pressure contacts, such as, for example, vacuum circuit breakers and certain types of switchgear, require a contact pressure spring which, in the closed position of the switch, maintains the pressure required for the passage of the current between the contacts independently of the other springs of the above described driving mechanism. The contact pressure spring is located at a point in the drive mechanism whereat driving forces are transmitted to the pressure contacts and is pre-tensioned in correspondence to the magnitude of the minimum contact pressure required. In the closing process, this pre-tension is overcome when the pressure contacts touch each other, thereby resulting in a steep rise in the force-vs-distance curve of the driving mechanism. Subsequent to overcoming the pre-tension of the contact pressure spring, the spring is further cocked by the so-called "follow-on" stroke, so that the desired or required contact pressure is still present in case the contacts are worn or burn-off occurs.
It is customary to design the aforesaid driving mechanisms so that their motion cycles are such that the metallic contact of the pressure contacts takes place toward the end of the closing movement. As a result, the contact pressure is supplied only during a relatively small portion of the total closing motion or stroke.
It is an object of the present invention to provide an improved driving mechanism of the aforesaid type.